1. Field of the Invention
The present invention relates to an active matrix liquid crystal display, and particularly, to that suitable for a projection liquid crystal display.
2. Description of Related Art
FIG. 1 is a schematic view showing an example of an active matrix, LCD (liquid crystal display) according to a related art. The LCD has column electrodes D1 took for display signals and row electrodes G1 to Gm for scanning. The row scan-electrodes G1 to Gm are orthogonal to the column signal-electrodes D1 to Dk. At each intersection of the column signal-electrodes and row scan-electrodes, a pixel PIX is formed. The pixels PIX are arranged in a two-dimensional matrix.
The column signal-electrodes D1 to Dk are driven by a column signal-electrode driver 1 having a horizontal shift register 2 and a group of switches SW. The shift register 2 has output stages connected to control terminals of the switches SW, respectively. Input terminals of the switches SW are commonly connected to a display signal (SIG) input terminal. Output terminals of the switches SW are connected to the column signal-electrodes D1 to Dk, respectively. The related art of FIG. 1 has k column signal-electrodes D1 to Dk, and therefore, there are k switches SW and the shift register 2 has k output stages.
The shift register 2 receives a horizontal start signal HST and a horizontal clock signal HCK from a timing signal generator (not shown). The output stages of the shift register 2 sequentially provide ON pulses to the control terminals of the switches SW, to sequentially turn on the switches SW and sequentially apply display signals SIG through the display signal input terminal to the column signal-electrodes D1 to Dk.
The row scan-electrodes G1 to Gm are driven by a row scan-electrode driver 3 having a shift register. The shift register has output stages connected to the row scan-electrodes G1 to Gm, respectively. The shift register receives a vertical start signal VST and a vertical clock signal VCK from a timing signal generator (not shown) and sequentially applies row select pulses to the row scan-electrodes G1 to Gm.
FIG. 2 shows one of the pixels PIX formed at the intersections of the column signal-electrodes D1 to Dk and row scan-electrodes G1 to Gm. The pixel PIX consists of a switching transistor Tr, a supplementary capacitor Cs, a display electrode (not shown), and a liquid crystal module (LCM). When a row select pulse is supplied to a row scan-electrode G connected to the pixel PIX, the switching transistor Tr of the pixel PIX as well as the switching transistors of the other pixels connected to the same row scan-electrode G turn on to receive display signals through the column signal-electrodes D1 to Dk. In the pixel PIX shown in FIG. 2, the display signal supplied a column signal-electrode D is stored in the capacitor Cs through the transistor Tr, and at the same time, drives the LCM. The capacitor Cs holds a liquid crystal drive voltage for an OFF period of the transistor Tr, to drive the LCM at high duty.
According to the related art, each pixel PIX has the switching transistor Tr and the supplementary capacitor CS to hold a display signal voltage. Namely, the related art employs a hold-type display method that holds a signal voltage representative of display information for nearly a whole frame. This method fundamentally has the following problems:
(1) Inferior Dynamic Image Resolution Due to Human Vision
The human vision works like a time-response filter that causes a delay when responding to a stimulus. A video device reproduces moving images by speedily displaying many frames of still images that are slightly different from one another. These still images produce after images on the human vision, and therefore, the human vision senses that the object is moving. The active matrix LCD employing the hold-type display method continuously displays a first frames image up to a moment to display a second frame image. As a result, the human vision sees an afterimage of the first frame image over the second frame image. This results in blurring the second frame image and deteriorating dynamic image resolution.
(2) Applied Voltage and Liquid Crystal Response
Response of liquid crystals is dependent on the cell gap, viscosity, elastic constant, and other characteristics of the liquid crystals. In particular, the response of liquid crystals delays in a halftone region where a voltage applied to the liquid crystals is above a threshold voltage. Such a delay in the response of liquid crystals deteriorates dynamic image resolution.